Video amplifier



Jan. 13, 1942.- n. E. FOSTER 2,269,654

VIDEO AMPLIFIER Filed May 22, 1940 5 1 h A" 72 4 l R 3 1 2 c' 'fi e- L OUTPUT l L :g ""r g; f a 8 7 l l GAIN FREQUENCY lNVENTOR ATTORNEY Patented Jan. 13, 1942 UNIT-ED OFFICE Dudley EQFoster', South Orange, N. J assignor to Radioicorporation of America, a corporation of I Delaware Application May 2-22, 1940, Serial no. 336,551 5.Claims. (Cl. 179-171) The present invention relates to -wide band video frequency amplifiers and particularly to amplifiers of the resistance-coupled-type-adapted to amplify picture signals in television'systems.

In video amplifiers in which resistance coupling is employed it is knownto minimize the effectof the inherent: plate-to-cathode capacity at high frequencies by v an inductance called a peaking coil, in series with "the" load resistor. An analysis and the design of such amplifiers are described by Seeley and Kimballin the RCA -Re view for January, 1939, at page '290. Byin-Q creasing the load resistance increased low frequency gain can be obtained-butthis-requires a;

larger peaking coil which 'in' turn, results: in a high frequency peak. Itrisfithereforethemain object of the present invention to associate-With the peaking coil a damping circuitwhichis'vari able with frequency and thusprovidean amplii fler which has a substantially fiat frequency reg sponse over a comparativelywiderange offre-fv quencies extending approximately "between 60.

cycles/second and 5.0 megacycles/secon'd.

Other objects; features and advantageszof' my invention will appear, from the following descripi tion taken in connection with the accompanying drawing in which Fig. 1 is a schematic circuit diagram of an amplifier embodying my invention, and Fig. 2 are response curves which will serve to explain the invention.

Referring now to Fig. 1 the amplifier system comprises the amplifier tubes T1 and T2, each of approximately 40% higher than the high frequencyend-of the range of frequencies to be amplified. Coupled to the coil L1 by mutual coupling' M is thedamping circuit consisting of the coil L2 and the shunt resistance R2,

The plate 5 of the tube T1 is coupled to the grid l'fl of tubeTz through the usual coupling condenser l6.

When the values of C, R1 and L1 comprising the plate load of a substantially constant current tube are so chosen that o 2OJQL1 wo' being equal to 2H0 and in being the top frequency of the range to-be transmitted, uniform impedance-is obtained up to In, and hence uniform gain. "This condition or response is represented' by the curve a of Fig. 2. If resistor R1 be 26 increased-without change in Cor L1 the low frewhich preferably is of the screen grid or pentode type which are substantially constant current tubes. The tube T1 includes an indirectly heated cathode l, a control grid 2, a screen grid 3, a suppressor grid 4 and an anode 5. The control grid may be biased negatively a suitable amount by means of the usual network consisting of resistor 1 and shunt condenser 8.

The tube T2 also includes an indirectly heated cathode 9, a control grid ill, a screen grid II, a

suppressor grid l2 and an anode Hi. The control grid l0 may be negatively biased, as in the case of tube T1, by means of the resistor-condenser network l4l5, the bias voltage being applied through a grid resistor Rg.

In the specific embodiment of the invention illustrated, operating voltage is applied to the anode 5 of the tube T1 from a suitable source of potential B+ through a coil L1, commonly known as a peaking coil, and an anode or load resistor R1. that it resonates with the inherent plate to cathode capacity C (shown dotted in the drawing) The value of the peaking coil L1 is so chosen" quency gain is increased but high frequencies drop-off as shown by the curve b. Increasing the value "of'Li will" again produce uniform response but value of f0 is decreased and the amplifier will not have fiat gain over as wide a frequency range.

If L1 be made larger than a peak occurs towards high frequencies as shown by curve 0, the frequency range still being decreased over that of curve 0. because the resonant frequency is lowered.

Now by coupling the circuit L2R2 to L1 it acts to vary the inductance of the load circuit in response to frequency. If R2 is small compared to L2 it will decrease L2 by the factor thus raising the resonant frequency as the applied frequency is increased by virtue of the change in effect of the coupled circuit. Resistor 7 should be resonant to a frequency higher than the desired uniform response range.

As has been explained, circuit LzRz acts to change the effective inductance in the plate circuit of T1. At low frequencies the effect of L2R2 is negligible but since it increases as the square of the frequency the effect becomes substantial at higher frequencies. The magnitude of variation of eifective inductance maybe changed by adjustment of either M or L2. If R: is not made small in comparison with the reactance of L2 at the higher frequencies it is desired to amplify, it too varies the effective inductance.

The action of circuit L2R2, as has been explained, is to change the effective inductance in the load circuit of T1 as a function of applied frequency and also because of R2 reflected into the plate circuit, to change to somerdegree the resistance component of the load circuit. The addition of the adjustable parameters permits a greater degree of freedom in the design of the amplifying stage to accomplish the desired results of wide band and high load impedance.

While I have shown and described a preferred embodiment of the invention, it will be understood that modifications and changes may be made without departing from the spirit and scope of the invention, as will be understood by those skilled in the art.

What I claim is:

1. A circuit arrangement for the uniform amplification of a wide range of frequencies, comprising an amplifier, a load circuit connected to the plate of the amplifier comprising a resistance and an inductance which serves as a peaking coil, a second circuit comprising at least inductance and resistance, the second circuit being inductively coupled to the inductance of the load circuit whereby the effective value of said load circuit is made to vary with variation of applied frequency.

2. An amplifier of high frequency energy comprising a load circuit connected to the plate of plifier, and a damping circuit mutually coupled to the output circuit inductance and variable with frequency for compensating for high frequency attenuation, said damping circuit comprisingan inductance and a shunt resistance, said latter inductance having mutual coupling with the output circuit inductance.

4. An amplifying system comprising a source I of input frequency varying from extremely low to extremely high frequency, an amplifier, an output load circuit for said amplifier comprising the series connection of a resistor and an inductive impedance, and a parallel combination of an inductance and a shunt resistor in magnetically coupled relation with the inductive impedancefor compensating for high frequency attenuation whereby the response characteristic of the amplifier is substantially uniform over a wide range of frequencies.

5. An amplifier system comprising a source of input frequency varying from a frequency of at least as low as cycles per second to a frequency at least as high as 5.0 megacycles per second, a pair of amplifier tubes, a resistor in the output circuit of the first amplifier, -a resistor in the input circuit of the second amplifier, a coupling condenser connected between said output and input Cil'CllitS,- an inductance connected in series with said output resistor, and a damping circuit comprising an inductance and a shunt resistor mutually coupled to the output circuit inductance, their impedance values being so related as to compensate for attenuation of the output voltage due to high frequency impressed voltage.

DUDLEY E. FOSTER. 

